The present invention relates to an electron gun for a cathode ray tube which is able to produce excellent beam spot shapes over the entire area of the screen independently controlling corrections for astigmatism and field curvature accompanying the electron beam deflection.
For an electron gun of the type used for a cathode ray tube, such as a television picture tube or a display tube, it is necessary to control the beam spot shape properly in accordance with the degree of the deflection angle in order to obtain a high resolution with excellent focus characteristics at all times over the entire area of the screen.
Electron guns of this kind have been disclosed, for example, in our Japanese Patent Laid-Open No. 72546/1990. The electron gun disclosed in this published Japanese application is provided with a first electrode unit (three-electrode unit) which generates a plurality of electron beams and directs these electron beams toward the screen along initial paths parallel to each other in one horizontal plane, and a second electrode unit which constitutes a main lens for focusing the aforesaid electron beams respectively on the screen. A focusing lens unit adjacent to the accelerating electrode of the two electrodes forming the main lens, to which the highest voltage is applied, comprises two electrode members. A first electrode member is adjacent to the accelerating electrode and is provided with an electron beam passage hole in an end surface thereof facing the second electrode member. Flat plate electrodes electrically connected to the first electrode member and sandwiching vertically the electron beam passage hole provided in the first electrode member are arranged to extend into the inside of the second electrode member through the single opening provided in the end surface of the second electrode member facing the first electrode member, which end surfaced faces the aforesaid flat plate electrodes with a constant interval. Further, the electron gun is so configured that an electrode plate having electron beam passages of the same diameter, extending in the direction parallel to the aforesaid horizontal plane, is electrically connected to the second electrode member. A voltage which varies in synchronism with the deflection required for scanning the plurality of electron beams on the screen is applied to the first electrode member.
In the above-described electron gun, in which the focusing lens adjacent to the accelerating electrode comprises a first electrode member and a second electrode member, a nonaxisymmetric electron lens (i.e., not circular at its cross section) is formed between the first electrode member and the second electrode member. Then, by applying to the first electrode member a voltage which varies in synchronism with the electron beam deflection, the cross-sectional shape of the electron beam is deformed to correct the astigmatism accompanying the deflection; and, at the same time, by providing the first electrode member adjacent to the accelerating member, the lens power of the main lens is varied in synchronism with the electron beam deflection to correct the field curvature in the peripheral part of the image on the screen.
However, in the above-mentioned structure, the correction of the field curvature is performed by only one main lens. Therefore, in order to balance the effect of the astigmatism correction by the electron lens formed of the first electrode member and the second electrode member of the focusing lens and the effect of the above-mentioned field curvature correction by the main lens, the astigmatism correction sensitivity should be reduced to match the astigmatism correction effect with the field curvature correction effect because the above-mentioned field curvature correction effect depends on the main lens. As a result, the application of a high dynamic voltage is required for the peripheral part of the screen.
In practice, however, because of the restriction imposed upon the circuitry of a television set, the applicable voltage should be lower than the voltage required to obtain an excellent image quality in the peripheral part of the screen. Thus, a problem is encountered in the prior arrangement in that the correction of astigmatism and field curvature cannot be performed sufficiently for the peripheral part of the screen, making it difficult to obtain an excellent image quality in the peripheral part of the screen.